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The present invention relates to an electrolytic cation exchange membrane. More particularly, the present invention rela-tes to a cation exchanye membrane suitable for use in the electrolysis oE water or an aqueous so]ution, such as an aqueous acidic or alkaline solu-tion, an aqueous alkali metal halide solu-tion or an aqueous alkali me-tal carbonate solution.
As a process for producing an alkali metal hydr-oxide and chlorine by the electrolysis of the above-men-tioned aqueous solution, particularly an aqueous solu-tion of an alkali metal chloride, the diaphragm method has now re-placed the mercury method to prevent environmen-tal pollu-: ....
tion. Further, in order to efficiently obtain an alkali metal hydroxide having high purity in high concentration, an ion exchange membrane has been employed.
However, for energy saving, it is desired -to im-prove the current efficiency and minimize the cell voltage in the electrolysis of this type. For this purpose, various methods have been proposed. However, this has not yet adquately been attained.
The present inventors have conducted extensive re-searches to provide an ion exchange membrane whereby the '`'~`~
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- 2 electrolysis of an aqueous solution can be conducted with high efficiency, and as a result, have succeeded in the development of an ion exchange membrane which is capable of adequately attaining the above-mcntioncd objcct.
Thus, the present invention provides an electrolytic cation exchange membrane which comprises a first film made of a fluorinated polymer having cation exchange groups and a second i~lm laminated thereon, made of a fluorinated polymer having carboxylic acid groups as its ion exchange groups and containing the following repeating units (A), (B) and (C) and having a smaller thickness and greater specific electric resistance than the first film;
(A) t CF2--CXX'~
(B) ~CF~--CX~
O -Rf (C) tCF2--CX~
Y- COOM
where each of X and X' is -F, -Cl or -CF3, Rf is a perfluoroalkyl group having from 1 to 10 carbon atoms, M is hydrogen or an alkali metal and Y is selected from the group consisting of ~CF2 )x ' 20 -o~CF2~C ~ ~ --CF2-- CF~ andtO--CF--CF2~OtCF~
Z Z Z
where each of x and y is an integer of from 0 to 10 and Z is C~ -F or Rf as defined above, provided that B/~+B+C~ (molar ratio) and C /~+B~C) (molar rat;o) are from 0. 01 to 0. 3 and from 0. 05 to 25 o. 5, respectively .
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Now, the present Inventlon wlll be descrlbed In detall wlth reference to the preferred embodlments.
The catlon exchange membrane of the present Inventlon havlng the above-mentloned constructlon, glves a superlor perfor-mance, I.e. hlgh current e~fIclency and low cell voltage, In the electrolysls. Thls Is attrlbutable to the fact that the second fllm constltutlng the catlon exchange membrane oF the present Inventlon Is macle of a fluorlnated polymer havlng carboxyllc acld groups and contalnlng the above-mentloned repeatlng unlts (A), (B) and (C) and It Is thereby posslble to readlly prepare an extremely thln fllm havlng a thlckness of e.g. from 5 to 40 ~ m, by e.g. extruslon moldlng even when the catlon exchange capaclty Is relatlvely small, whlch prevlously was dlfflcult wlth a fluo-rlnated polymer of thls type.
Namely, the lon exchange capaclty requlred for a fluo-rlnated polymer havlng carboxyllc acld groups whlch constltutes a catlon exchange membrane may vary dependlng upon the concentra-tlon of the alkall metal hydroxlde to be produced by the elec-trolysls, but the lon exchange capaclty Is usually requlred to be relatlvely small. In such a case, the speclflc electrlc resls-tance of the fluorlnated polymer must be hlgh. In the catlon exchange membrane of the present Inventlon, the fluorlnated poly-mer havlng a small lon exchange capaclty and a hlgh speclflcelectrlc reslstance whlch govern the performance of the membrane Is formed Into a fllm havlng an extremely small thlckness, and such a thln fllm (the second fllm) Is lamlnated on a fllm (the flrst fllm) havlng a larger thIckness than the second fllm and made of a fluorlnated polymer havlng a large lon exchange capac-lty and a small speclflc electrlc reslstance, whereby superlor performance wlth respect to the current effIclency and the cell voltage In the electrolysls Is obtalnable wlthout Impalrlng the mechanlcal strength of the membrane.
In the present inventlon, the flrst fllm of the fluorl-\
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nated polymer havlng catlon exchange groups should preferably be made to ~lave an ion exchange capaclty as large as posslble and a speclflc electric reslstance as small as posslble so long as ade-quate rrlechanlcal strength can thereby be malntalned. The catlon exchange groups of the flrst fl Im may be any groups such as sul-fonlc acld groups, carboxyllc acld groups, phosphonlc acld groups or hydroxyl groups.
The content of the catlon exchange groups In the flrst fllm Is selected to have an l'on exchange capacity of from 0.5 to 4.0 meq/g dry resln, preferably from 0.8 to 2.0 meq/g dry resln-so that the speciflc electrlc reslstance becomes smaller than In the second. In a case where weak acld groups such as carboxyllc acld groups, phosphor'lc acld groups or hydroxyl groups are used as the catlon exchange groups, a flrst flIm lon exchange capaclty hlgher than that of the second fllm Is selected.
Varlous klnds of fluorlnated polymers may be used for the preparatlon of the flrst flIm. Among them, polymers havlng the followlng repeatlng unlt (a) and tb) are preferably used.
(a) --tCF2 CXX'--t-(b) --~CF2 CX
YA
where X, X' and Y are as deflned above, and A Is -S03M, -COOM, -P03M?, or -OM (where M Is as deflned above). The molar ratlo of (a)~(b) Is selected to obtaln an lon exchange capaclty wlthln the above-mentloned range.
The fluorinated polymers are preferably perEluoropolymers.
Preferred perfluoropolymers include a copolymer of CF2=CF2 and CF2=CFOCF2-CF(CF3)OCF2CF2SO2F, a copolymer of CF2~CF2 and CF2=CFO(CF2)2 5SO2F, a copolymer of CF2=CF2 and CF2=CFO(CF2~2 5COOCH3 and a copolymer of CF2=CF2 and CF2=CFOCF2CF (cF3)ocF2cF2coocH3 .
In the present invention, the fluorinated polymer having 10 carboxylic acid groups as ion exchange groups which consti~utes the second film, has the following repeating units (A), (B) and (C):
(A) t CF2--CXX' ~
(B) tCF2--ICXt and O -Rf 15 (C) tCF2--CX ~
Y - COOM
where X, X', Y, Rf and M are as defined above. Among the fluroinated polymers, perfluoropolymers are preferred. As such perfluoropolymers, there may be mentioned a three component copolymer of CF2=CF2, 20 CF2=CFORf (where Rf is a perfluoroalkyl group having from 1 to 3 carbon atoms) and CF2=CFO(CF2)1 4COOCH3 or a three component copolymer of CF2=CF2, CF2=CFORf ~where Rf is as defined above) and CF2=CFOCF2CF(C~3)O(CF2)1 3COOCH3.
The ratio of the repeating units (A), (B) and (C) in the is (J~-~cr~n ~) cl f, B 25 fluorinated polymer is important since the ratio e~7s the property of the film. Namely, B /A~B+C (molar ratio) is nttributable to the fabricability of the film and is preferably within a range of from 0.01 to 0.3~ more preferably from 0.03 to 0.2. lf the molar ratio is small, the improvement in the film fabricability tends to be small. On the other hand, if the molar ratio is too grent, the mechanical strength of the film tends to decrease. ~Vhereas, . ~
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C/A+B~C (molar ratlo) Is determlnatlve to the lon exchange capac-lty of the flIm, and the ~olar ratlo may be selected dependlng on the deslred lon exchange capaclty and upon the concentratlon of the alkall metal hydroxlde to be produce~ by the electrolysls.
The molar ratlo of 0.05 to 0.5 Is usually selected to glve an lon exchange capacl-ty of from 0.5 to 2;0 meq/g dry resln, preferably from 0.7 to 1.5 meq/g dry resln, further preferably from 0.8 to 1.~ meq/g dry resln.
The fluorlnated polymers for the flrst and second fllms may be prepared by varlous methods. Furtner, these fllms may optlonally be relnforced by a woven fabrlc such as cloth or net, a non-woven fabrlc or a flbrll made of a fluorlnated polymer such as polytetrafluoroethylene or by a metal mesh or a foramlnous metal sheet.
In order to maxlmlze the performance of the lon exchange membrane by the comblnatlon of the flrst and second fllms of fluorlnated polymers accordlng to the present Inventlon, the fIrst f~lm Is preferably made to l1ave a thlckness of from 100 to 700~ ~ m, more preferably, from 150 to 500~ m, further prefer-ably 150 to 300~ m and the second fllm Is preferably made to have a thlckness of from 5 to 50 ~jm, more preferably from 150 to 300 ~ m, further preferably from 10 to 30 ~ m, and the ratlo of thlckness of the fIrst fllm to the thlckness of the second fllm Is preferably selected to be from 1.0 to 50, more preferably from 2.0 to 30, further preferably from 5 to 20. In a case where the second fllm Is extremely thln, the above-mentloned relnforclng materlal Is preferably Introduced to the flrst fllm.
For the lamlnatlon of the flrst and second fllms, an optlonal method may be employed, and In any case, the two fllms must be Integrated by the lamlnatlon. For Instance, the lamlna-tlon Is carrled out by presslng them preferably at a temperature of from 100 to 350C under pressure of from 0.5 to 100 kg/cm2.
In the present Inventlon, In some case, a lamlnate o f two or more dlfferent k I nds of fllms may be use~ for the f I rst fl Im or the second fllm or for both the flrst and secon~ fiIms for the laml-nation. When the two fllms are larnlnated, the respectlve catlon exchange groups should not be subJected to decomposltlon for Instance, In the case of carboxyllc acld groups, they should preferably take a form of an acld or an ester at the tlme of lam-lnatlon, and In the case of sulfonic acld yroups, they should preferably take a form of -S02F at the tlme of lamlnatlon. The thlcl<ness of the catlon exchange membrane obtalned by the lamlna-10 tlon Is preferably from 80 -to 500~ m, more preferably from 100 to 300 ~ m.
The cation exchange membrane of the present Inventlon thus obtalned by the lamlnatlon of the flrst and second fllms, exhlblts superlor performance by Itself. However, If deslred, a gas and llquld permeable porous layer contalnlng catalytlcally actlve partlcles ~U.SI. Patent No. 4224121) or a gas and llquld permeable porous layer contalnlng catalytlcally Inactlve part-Icles (U.l<. PublIshe~d Patent ApplIcatlon No. 2064586) may be pro-vlded on one slde or both sldes of the membrane to furtherImprove Its performance.
The catlon exchange membrane of the present Inventlon Is useful for the electrolysls of varlous aqueous solutlons, par-tlcularly an aqueous alkall metal chlorlde solutlon as mentlonedabove.~ For Instance, when used for ~he electrolysls of an aque-ous alkall metal chlorIde solutlon, the catlon exchange membrane of the present Inventlon Is dlsposed so that the flrst fllm faces the anode slde and the second fllm faces the cathode slde, whereby the catlon exchange membrane of the present Inventlon exhlblts the maxlmum performance.
The electrolysls of an aqueous alkall metal chlorlde solutlon wlth use of the catlon exchange membrane of the present Inventlon, may be conducted under such known condltlons as dls-closed In Japanese Unexamlned Patent Publlcatlon No. 112398/1979 .,, ~23~
publIshed September ~, 1979 to General Electrlc Company. For Instance, whlle supplylng an aqueous alkall metal hydroxlde solu-tlon of preferably from 2.5 to 5.0 N to the anode compartment and water or a dlluted alkall metal hycirox I de to the cathode compart-ment, the electrolysls Is conducted preferably at a temperatureof from 80 to 120C .at a curren-t denslty of form 10 to 100 A/dm2.
In such case, It is advlsable to mlnlmlze metal lons such as cal-clum or magneslum in the aqueous alkall metal hydroxlde solutlon as such metal lons tend to lead to degradatlon of the catlon exchange membran,e. Further, In order to mlnlmlze the generatlon of oxygen at the anode, an acld such as hydrochlorlc acld may be added to the aqueous alkall metal hydroxlde solut,lon.
The electrolytlc cell used In the present Inventlon may be a monopolar or blpolar type so long as It has the above-men-tloned structure. The electrolytlc cell used In the electrolysls of an aqueous solutlon o-f an alkall metal chlorlde, Is made of a materlal belng reslstant to the aqueous solutlon of the alkall metal chlorlde and chlorlne such as valve metal llke tltanlum In the anode compartment and Is made of a materlal belng reslstant to an alkall metal hydroxlde and hydrogen such as Iron, stalnless ~, steel or nlckel In the cathode compartment.
When the electrodes are placed In the electrolytlc cell 2~ of the present Inventlon, they may be dlsposed to contact the lon exchange membrane, or they may be placed wlth an approprlate space from the lon exchange membrane.
In the foregolng, the use of the membrane of the pre-sent Inventlon has been descrlbed prlmarlly wlth respect to theelectrolysls of an aqueous alkall metal chlorlde solutlon. How-ever, It should be understood that the membrane of the present Inventlon is llkewlse appllcable to the electrolysls of water, a halogen acld (hydrochlorlc acld or hydrobromlc acld) or an alkall
3~ metal carbonate.
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Now, the present Inventlon wlll be descrlbed wlth ref erence to Examples whlch are provlded for the purpose of Illus-tratlon and are not Intended to llmlt the present Inventlon.
Into a 10 ~ stalnless steel pressure reactor, 6500 g of delonlzed water, 13 g of C8F17COONH4, 32.4 g of Na2~1P04.12H20, 19.5 g of NaHzPO4.2H20 and 1.7 g of (NH4)2S203 were fed and then 1300 9 of CF2=CFO(CF2)3COOCH3 was fed. After thoroughly deaerat-lng wlth llquld nltrogen, the temperature was ralsed to 57C, and 11.0 kg~cm2 of tetrafluoroethylene was Introduced and reacted.
Durlng the reactlon, tetrafluoroethylene was contlnuously Intro-duced Into the system to malntaln the pressure at 11.0 kg.cm2.
4.5 hours later, the reactlon was termlnated and the obtalned latex was flocculated by means of concentrated sulfurlc acld.
The polymer thereby obtalned was thoroughly washed wlth water, then treated In methanol at 65C for 16 hours and drled to obtaln 1520 g of a copolymer havlng an lon exchange capaclty of 1.44 meq/g. To the copolymer, 2.7% by welght of PTFE partlcles (supplled under the trademark Teflon 6J) were added and the mlx-ture was kneaded ~y kneadlng rolls to fIbrllate PTFE and then extruded at 230C to form a fllm havlng a thlckness of 260 ~ m.
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, Into the same reactor, 6500 g of deioni~ed water, 13 g o~
CgF17COONH4, 32-5 g of Na2HPO4-12H2o, 19.5 g of NaH2PO~-2H2O, 1.7 g of (NH4)2S2O3 and 0.~6 g of isopropanol were fed and then 845 g of CF2=CFO(CF2)3COOCH3 and 450 g of CF2=C~OC3F7 were fed.
5 After deaerating wlth liquid nitrogen, the temperature was raised to 57C and 12.4 kg/cm2 of tetrafluoroethylene was introduced and reacted. During the reaction, tetrafluoroethylene was introduced from outside to maintain the pressure at 12.4 kg/cm2. 4.5 Hours later, the obtained latex was treated in the same manner as above, whereby 1290 g of a three component copolymer having an ion exchange capacity of 0.86 meq/g was obtained. The three component copolymer was extruded at 230C to form a thin film having a thickness of 20 llm.
Then, the two types of fflms were laminated at 230C by means of rolls to obtain a double-layered membrane. The membrane was hydrolyzed in an aqueous solution containing 12% by weight oiE sodium hydroxide. With use of this membrane, electrolysis was carried out in the following manner.
By means of a small ~cale electrolytic cell having an effective membrane surface area of 0. 25 dm2, an anode of RuO2 coated on Ti expanded metal, a cathode of active nickel coated on Fe expanded metal and an electrode distance of 3 mm, an electrolytic test was carried out at 90C under a current density of 20 A/dmP while supplying 300 g/ Qof NaCl and water to the anode compartment and the cathode compartment, respectively. As the results, the cell voltage was 3.07 V and the current efficiency was 96.0% when the concentration of the formed sodium hydroxide was 22% by weight.
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EXAMPLE 2:
The two types of films obtained in Example 1 were laminated at 230C by means of rolls.
Then, a mixture comprising 10 parts by weight of silicon carbide 5 powder having an average particle size of 2 ,um, 1 part by weight of polytetrafluoroethylene particles, 0.3 part by weight of methyl cellulose (a 2% aqueous solution), 14 parts by weight of water, 2 parts by weight of cyclohexanol and 1 part by weight of cyclo-hexanone, was kneaded to obtain a paste. The paste was applied 10 by screen-printing onto the surface on one side (the three component copolymer layer having an ion exchange capacity of 0.86 meq/g) of the above- mentioned laminated membrane, then dried and soliidified .
The amount of the deposition of the silicon carbide was 1.0 mg per 1 cm2 of the membrane surface. Then, a paste prepared in the same 15 manner as above except that oxidized zirconia having an average particle size of 7 ~m was used, was applied on the surface on the other side (the copolymer layer having an ion exchange capacity of 1. 44 meq/g) of the laminated membrane, then dried and solidified t obtain a deposition of 0. 95 mg/cm2 . The membrane thus prepared 20 was hydrolyzed in an aqueous solution containing 12% by weight of sodium hydroxide, and electrolysis was conducted in the same manner as in Example 1. As the results, the current efficiency was ~6.0 and the cell voltage was 2.87 V to obtain sodium hydro~de at a concentration of 22~ by weight. On the other halld, the current efficiency was 92.5% and the cell voltage was 2.98 to obtain sodium hydroxide at a concentration of 35% by weight.
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EXAMPLE 3:
Into a 10 Q stainless steel pressure reactor, 6500 g of deionized water, 13 g of C8F17COONH4, 32.5 g of Na~HPO4 12H2O, 19.5 g of NaH2PO4-2H2O, 5.2 g of (NH4)2S~O3 and 2.6 g of NaHSO3 were fed, S and then 1950 g of CF2=cFo(cF2)3coocH3 was fed-After thoroughly deaerating with liquid nitrogen, the temperature was raised to 40C, and 5.1 kg/cm2 of tetrafluoroethylene was intro-duced and reacted. During the reaction, tetrafluoroethylene was continuously introduced to the system to maintain the pressure 10 at 5.1 kg/cm2. 9.5 Hours later, the reaction was terminated, and the obtained latex was flocculated by means of concentrated sulfuric acid . T he polymer thereby obtained was thoroughly washed with water and then treated in methanol at 65C for 16 hours, whereby 1600 g of a copolymer having an ion exchange capacity of 1.80 meq/g 15 was obtained. To the copolymer, 5.59~ by weight of PTFE particles (Teflon 6~) was added, and the mixture was kneaded at 130C by means of kneading rolls to fibrilate PTFE and then extruded at 230C
to form a film having a thickness of 200 l~m. To this film, the film of the three component copolymer having an ion e~;change capacity 20 of 0. 86 meq/g which was obtained in Example 1, having a thickness of 30 1Im, was laminated at 230C by means of rolls to obtain a double-layered membrane. Then9 in the same manner as in Example 2, 0. 96 mg/m2 of zirconium oxide was deposited on one side (the polymer layer having an ion exchange capacity of 1.80 meq/g) of the double-25 layered membrane and 1. 02 mg/m2 of silicon carbide was deposited on the other side (the three component copolymer layer having an ion exchange capacity of 0~86 meq/g) of the membrane.
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Then, the membrane thus prepared was hydrolyzed in an aqueous solution containing 12% by weight of sodium hydroxide, and electrolysis was conducted in the same manner as in Example 1. As the results, the current efficiency was 95.5% and the cell voltage was 2.69 V to 5 obtain sodium hydroxide at a concentration of 22% by weight.
EXAMPLE 4:
A polymer having an ion exchange capacity of 1.18 meq/g, obtained by copolymerizing tetrafluoroethylene with CF2=CFOCF2CFO(CF2)3COOCH3, was formed into a film having a thickness of 260 llm. To this film, the film of the three component copolymer having an ion exchange capacity of 0.86 meq/g which was obtained in Example 1, having a thickness of 10 llm, was laminated.
The membrane thus obtained was hydrolyzed in an aqueous solution 15 containing 12% by weight of sodium hydroxide, and the ohmic loss of the membrane was measured. The ohmic loss of the membrane was 0.35 V.
EXAMPLE 5:
A polymer having an ion exchange capacity of 1.1 meq/g, 20 obtained by copolymerizing tetrafluoroethylene with CF2=CFOCF2CFOCF2CF2SO2F, was formed into a film having a thickness of 200 1lm, followed by the lamination with the film of the same three component copolymer used in Example 4. The hydrolysis and the measurement of the ohmic loss of the membrane in the same 25 manner as in Example 4. The ohmic loss of the membrane was 0.29 V.
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COMPARATIVE EXAMPLE 1:
A polymer havlng an lon exchange capaclty of 0.86 meq/g was prepared In the same manner as In Example I except that Instead of the three component polymer havlng an lon exchange capaclty of 0.86 meq/g, only te-trafluoroethylene and CF2=CFO
(CFz)3COOCH3 were copolymerlzed. The polymer was extruded at 230C in an attempt to obtaln a fllm. However, when one attempted to obtaln a fllm havlng a thIckness of a-t most 100 ~ m, the fllm tended to have holes so that It was Imposslble to obtaln a thln fiIm as in the case of the three component copolymer.
COMPARATIVE EXAMPLE 2:
1~ The ohmlc loss of the membrane cornposed of dlfferent polymers constltutlng each layer of the lamlnated membrane In Example 1 was shown In the followlng Table.
lon exchallge Thickness of Ohmic loss capaCitY (me~i) the mcmbr~ e(~J) (V) 1. ~ ~80 0. 19 0.8~ 280 1.0 or 2~ _ more , , . . .
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